Symbiosis In Rhizobia As An Evolutionary Strategy

Symbiosis in Rhizobia as an Evolutionary Strategy

With the immense number of organisms living on the biosphere, encounters occur constantly between organisms. No organism or species lives in complete isolation. Contact between organisms is inevitable and results in a variety of outcomes. Due to the large number, 5x1030, of bacteria living on planet earth, symbiotic relationships between bacteria and higher-order organisms are very common (Chem et al. 2003). In many cases, the relationship between bacteria and the higher-order organism is parasitic. The bacteria invade the host and utilize it for survival and reproduction, damaging and causing the premature death host in the process. Nonetheless, bacteria can prove beneficial to their hosts

Several hypotheses exist to explain why symbionts evolve to benefit rather than harm their hosts. One hypothesis suggests that symbiosis increases reproductive success because cooperation increases resources available to symbiont (Provorov. 1998). Reyes and Schmidt provided evidence supporting this hypothesis by showing that a marked strain of rhizobia reached a density of 3x106 cells/g in soil surrounding deteriorating a nodule, while only 2x104 cells/g were observed in soil not containing legumes (Reyes and Schmidt. 1979). This study indicates that carrying capacity for rhizobia is higher inside the plant then in the soil. Suggesting there must be some fitness advantage associated with symbiosis. Incorporation of symbiotic rhizobia in a legume increases the productivity of the plant due to the increased nitrogen levels made available to the plant through fixation by rhizobia (Provorov. 1998). This study does not concretely demonstrate that associations with legumes provide more resources to be used for reproduction. Although, one could rationalize that increased legume productivity implies more photosynthate production and therefore more resources made available to the rhizobium for possible use in its own reproduction. The evolutionary persistence of symbiosis suggests that symbiotic rhizobia leave more progeny then both rhizobia that only colonize the rhizosphere and parasitic rhizobia (Denison.